NASIRIYA, Iraq (CNN) -- Intense fighting broke out near this southern Iraqi city Tuesday morning, the third consecutive day that U.S. Marines have battled Iraqi forces here to establish control of a key north-south route.

The city has been the site of some of the heaviest fighting of the war. U.S. military sources said 10 Marines were killed in action and at least five others taken prisoner Sunday. (Coalition casualties)

The Marines are trying to secure control of the bridges over the Euphrates River, which are a key to the U.S. advance north toward the Iraqi capital, Baghdad.

CNN correspondent Art Harris is traveling with the 3rd Battalion, 2nd Marine Division, Task Force Tarawa. Harris said as the battalion drove on the main road, which crosses the key bridges, he saw seven Marine vehicles, including tanks, armored personnel carriers and Humvees, that were "lost."

Two of those vehicles were stuck in mud and abandoned, Harris said. Five others had been destroyed.

Marine sources said four of those vehicles were destroyed by Iraqis, but the fifth was "shot up" by a U.S. A-10 Thunderbolt warplane, an attack aircraft often used to destroy tanks.

The sources said it was unknown whether anyone was killed in the A-10 attack on the Marine vehicle.

As the Marine unit rolled into Nasiriya just before dawn, the main road was "wall-to-wall" with Marines, Harris reported. They were lying flat on the ground, M-16s at the ready, facing two directions. Tanks, Bradley fighting vehicles and other armored personnel carriers were positioned at every intersection, he said.

Harris said U.S. forces had put up constant fire for the three hours since they rolled through. The Marines used .50-caliber machine guns, mortars and heavy artillery.

Shots were being fired at the Marines from all around, and Harris said it was difficult to determine enemy positions.

Marines told Harris that they were seeing an apparent pattern developing, in which unarmed Iraqis wearing traditional robes see the U.S. forces go by, then disappear into their homes and emerge with rocket-propelled grenades and AK-47 assault rifles, which they then fire at the Marines.

The Pentagon lashed out against such unconventional tactics Monday, calling the attacks "deadly deception" that mark "serious violations of the laws of war." (Full story)

The Defense Department has announced the names of seven of the 10 Marines killed near Nasiriya on Sunday. They are: